Environ, RTrim, and FormatCurrency live in the reference library:
Visual Basic For Applications
You can find the reference library for any function by opening the object browser (View/Code shows code window. From code window View/Object Browser).
In the object browser, choose "<All Libraries>" and enter the function you're searching in the "Search Text" box. The function you searched for will show up in the search results, along with the reference library it lives in.